The present invention relates to processes for producing crystallized sodium bichromate and more particularly to an improvement of the classical procedure of preparing crystallized sodium bichromate dihydrate, Na.sub.2 Cr.sub.2 O.sub.7. 2H.sub.2 O, to permit production of a very pure product free from chlorine and vanadium impurities, such impurities arising either from the original chrome ore or from materials used in the treatment of such ore.
It is known to produce sodium bichromate by various methods. Industrially, the preparation is carried out starting with chrome-containing minerals of which the most usual is chromite. The ore is subjected to an alkaline oxidation to form sodium chromate, and the chromate is separated from the mineral gangue by lixiviation or leaching with water. The resulting sodium chromate solution is freed of alumina by precipitation using partial acidification to a pH of about 9. The solution or wash liquor of neutral sodium chromate obtained is converted to bichromate liquor by treatment with sulfuric acid. The insoluble sodium sulfate formed during the acid treatment is separated, and the remaining liquor is concentrated to cause crystallization of the sodium bichromate. One such process is shown in German Pat. No. 113,086, issued July 20, 1900.
The sodium bichromate obtained by the foregoing classical process is not pure. Chlorine and vanadium impurities are present, and these are particularly troublesome for uses in which the products derived from sodium bichromate are generally destined. These products are, for example, chromic acid and the basic sulfates of chromium. In particular, the presence of vanadium is totally prohibited in chromium sulfate utilized in tanneries because it causes the appearance of brownish stains on the tanned leather.
In the past the chemical methods to eliminate the vanadium contained in sodium bichromate or in its solutions have not been economical. It has been proposed to obtain vanadium-free sodium bichromate by subjecting the chromate solution to a special treatment for removing the vanadium, for example by a lime treatment which eliminates the vanadium in the form of insoluble calcium metavanadate. This treatment presents cost difficulties because it occasions a loss of chrome values in insoluble forms, especially increasing with high content of trioxide, C.sub.r O.sub.3, in the chromate wash.